Other References
William S. Coblenz: xe2x80x9cThe Physics and Chemistry of the Sintering of Siliconxe2x80x9dxe2x80x94Journal of Materials Science, Vol. 25, 1990, pp. 2754-2764
Brosnan, J, and Snow, B: xe2x80x9cRecovery of Silicon Metal from Drossxe2x80x9dxe2x80x94Minerals and Energy Research Institute of Western Australia, Report No. 194, Project No. M258, April 1998.
Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates to photovoltaic cells produced from silicon wafer sawing kerf. More specifically, the present invention relates to an improved process for the recovery of silicon from wafer sawing kerf and its application toward the fabrication of photovoltaic modules with improved light absorption and high fill factors.
2. Description of Prior Art
Photovoltaics (PV) technology is well established as a reliable and economical source of clean electrical energy. Having surpassed $1 billion in annual global sales, the photovoltaics industry is expected to add tens of GW/year of new generating capacity worldwide. In this context, future photovoltaic manufacturing facilities may be anticipated to produce on the order of 1 GW/year and will need to achieve a throughput on the order of 10 m2 of PV modules per minute.
The photovoltaics industry has been using reject material from integrated circuit (IC) polysilicon and single crystal production. Although annual sales of the worldwide silicon PV module industry are about 400 times smaller than those of the IC industry, the PV industry consumes about 10% of the worldwide polysilicon production. As the photovoltaics industry is growing faster than its large cousin the microelectronics industry, this material becomes rarer and more expensive. In the last six or seven years, the price of polysilicon has doubled, and shortages have occurred. If production of polysilicon does not increase substantially, shortages of this material may create an acute problem.
More than 98% of semiconductor-grade polysilicon is produced by the trichlorosilane (SiHCl3) distillation and reduction method, which is very energy intensive and produces large amounts of wastes, including a mix of environmentally damaging chlorinated compounds. About 80% of the initial metallurgical-grade silicon material is wasted during the process. In addition, semiconductor-grade polysilicon material far exceeds the purity requirements of the PV industry, and the cost is several times higher than what the PV industry can afford. It is obvious then that less complicated, less energy intensive, more efficient, and more environmentally benign methods need to be developed to meet the cost and quality requirements of the PV industry.
Most of the PV modules produced today are based on single-crystal silicon grown by the Czochralski (CZ) method. The CZ method begins by melting high purity silicon with a dopant in a quartz crucible. A small piece of solid silicon (the seed) is placed on the molten liquid in an inert gas atmosphere of about 1400 C. As the seed is slowly rotated and pulled from the melt, the surface tension between the seed and the molten silicon causes a small amount of the liquid to rise with the seed and cool into a single crystalline ingot with the same orientation as the seed. Crystal growth from silicon melt generates relatively few wastes. The main concern is the energy required and the amount of argon gas used during crystal growth.
After the silicon ingot is grown, it is sliced into wafers. Multiple wiresaw technology is now the preferred method of slicing large diameter ingots and the only viable technology for slicing 300 mm (12xe2x80x3) wafers. Wiresaw technology has helped trim wafer thickness to as little as 200 micrometers and to minimize sawing kerf, the layer of silicon about 250-280 micrometers thick, that is lost per wafer. Depending on wafer thickness, kerf loss represents from 25% to 50% of the silicon ingot material.
In the wiresaw process, an optionally diamond-impregnated steel wire, about 180 micrometer in diameter running over control spindles, is placed under high tension and pushed onto the silicon ingot while an abrasive slurry composed of 25 micrometer or less silicon carbide (SiC) particles in a mineral oil or glycol-base is fed to the cutting zone between the wire and the workpiece.
Slurry management is critical to maintain uniform wafer quality. For PV silicon wafer manufacturing, the silicon kerf content in the slurry can be as high as 30% by weight, while for electronics-grade wafer manufacturing, it is rarely allowed to exceed 10%. Wafer slicing is one of the most expensive process steps in silicon solar cell manufacturing, accounting for over 65% of the total wafer manufacturing costs due to the large quantities of consumables (stainless steel wire and abrasive slurry) and the kerf loss. Each single wiresaw machine may require $150,000-250,000 per annum in silicon carbide replacement and slurry disposal costs.
If a method could be developed to produce solar-grade polysilicon by purifying the sawing kerf of semiconductor-grade ingots, enough polysilicon would be generated for over 300 MW/year of crystalline-silicon solar cells, i.e., more than two times the requirements of the current silicon solar-cell production.
In accordance with the present invention the problems of the prior art are substantially overcome by providing an economical and environmentally benign method to recover crystalline silicon metal kerf from wiresaw slurries and to shape and sinter said recovered crystalline silicon kerf into thin-layer PV cell configurations with enhanced surface texture for metallization and reduced optical reflection losses.
It is a primary object of this invention to provide an economic, simple, energy and material efficient process to mass-produce PV cells.
An additional object of this invention is to provide a method to fabricate PV cells that have a thickness of 10 micrometers or less. Standard silicon wafers are 200-500 xcexcm thick, yet with improved optical designs, only 10 micrometers of silicon are required to capture all the available light. Hence one of the major technical hurdles the PV industry is trying to overcome is to economically produce thin-layer PV cells. At present this can only be achieved via sophisticated and costly silicon vapor deposition techniques. Since the granulometry of the silicon kerf particulates in the molding compound is in the sub-micrometer range, extremely thin wall geometries and design features can be achieved. Using the present invention PV cell thicknesses of the order of 10 micrometers and even less are attainable.
It is yet another object of the instant invention to provide a method of forming low-cost, high-quality front contacts on PV cells. Screen-printing is now the universally employed technique for contact formation. The problem with screen-printing, however, is that the throughput gains are attained at the expense of device performance. PV cell fill factorsxe2x80x94the maximum power generated by the PV cell divided by the product of open circuit voltage and short circuit currentxe2x80x94can be degraded by gridline resistance, contact resistance, and contact formation induced junction leakage and shunting. Hence gridline optimization using via fine line contacts can contribute to achieving high fill factors. By applying the present invention the front surface of PV cells can be fitted with molded-in microgrooves for optimized gridline metallization resulting in cells with higher fill factors.
Still another object of the present invention is to provide a method to increase PV cell efficiency, the latter being defined as the ratio of the electric power produced to the power of the incident light, or photons. The highest-efficiency silicon cell yet devised has a complex surface, consisting of inverted pyramids, resulting in reduced optical reflection losses. A significant benefit resulting from the combination of the fineness of the silicon kerf particulates in the molding compound and the shaping capabilities rendered possible by the present invention is the ability to achieve extremely complex surface textures contributing to the reduction of optical reflection losses. Such texture features can assume virtually any geometry that is moldable i.e. from which a molding core can be removed upon molding.
Yet another advantage of the present invention is the possibility to dope the silicon kerf molding compound with boron or phosphorus so as to obviate the need for post sintering doping operations.
Not applicable.